WASHINGTON (Reuters) - U.S. Federal Communications Commission Chairman Kevin Martin has proposed requiring Internet-based telephone services to offer 911 emergency services to customers by as early as the end of September, people familiar with the plan said on Wednesday.

After a few incidents where customers failed to reach emergency officials when they dialed 911, federal regulators are increasing pressure on companies to ensure those calls get routed and answered properly with location information.

The proposal would require companies like Vonage Holdings Corp. to route 911 calls directly to primary emergency lines within four months of the order being issued, the sources said, declining to be identified because the proposal is not a public record.

Martin has circulated the proposal so it could be voted at the agency's open meeting on May 19, the sources said. He would have to win the votes of two of the other three FCC commissioners for approval or work out a compromise with them.

An FCC spokesman had no immediate comment.

Companies are racing to offer Internet telephone service, known as Voice over Internet Protocol (VOIP), in part because it is cheaper to operate. It requires a high-speed Internet connection and subscribers can use it wherever they have that access.

Calls to 911 with traditional telephones provide emergency service dispatchers with the caller's number and address. In contrast, Voip providers do not all connect to the systems that route calls directly to emergency dispatchers.

Sometimes the 911 calls do not include location information and end up at office numbers that are not always answered.

Martin's proposal would cover those customers who use their Voip service in a single location as well as others who use the service in multiple places as long as they register that location, one source said.

It was not immediately clear how the FCC would ensure Voip providers have access to the equipment they need to route the calls to primary 911 call center lines or how the agency would enforce the requirements.

Vonage, the biggest U.S. Voip provider with more than 500,000 subscribers, has reached deals for 911 service in areas where Verizon Communications <VZ.N> and Qwest Communications International Inc. <Q.N> are the primary local phone carriers, the northeast corridor and most western states.

Vonage has been sued by two states where customers had trouble reaching 911 help, Texas and Connecticut. The state attorneys general accused the company of failing to adequately warn customers about the limitations of 911 with a Voip phone.